The "video history" of the crashed USAF ship makes it very clear that the planet is uninhabited when they "landed". I can understand how a race of apes develops - they had a bunch of them on board. I can understand how a race of humans develops - they are descendants of the original crew. What I don't understand is...where the heck did all the horses come from?
In order to star in the film, Tim Roth had to decline the role of Professor Severus Snape in "Harry Potter and the Philosopher's Stone" (2001). That role was eventually given to Alan Rickman.